This invention relates generally to gas generant materials such as used in the inflation of automotive inflatable restraint airbag cushions and, more particularly, to the enhancement of the rate at which such materials burn or otherwise react.
Gas generating materials are useful in a variety of different contexts. One significant use for such compositions is in the operation of automotive inflatable restraint airbag cushions.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an “airbag cushion,” that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Such systems typically also include one or more crash sensors mounted on or to the frame or body of the vehicle to detect sudden decelerations of the vehicle and to electronically trigger activation of the system. Upon actuation of the system, the cushion begins to be inflated or expanded, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as an “inflator.” In practice, such an airbag cushion is desirably deployed into a location within the vehicle between the occupant and certain parts of the vehicle interior, such as a door, steering wheel, instrument panel  or the like, to prevent or avoid the occupant from forcibly striking such part(s) of the vehicle interior. As a consequence, nearly instantaneous gas generation is generally desired and required for the effective operation of such inflatable restraint installations.
Various gas generant compositions have heretofore been proposed for use in vehicular occupant inflatable restraint systems. Gas generant compositions commonly utilized in the inflation of automotive inflatable restraint airbag cushions have previously most typically employed or been based on sodium azide. Such sodium azide-based compositions, upon initiation, normally produce or form nitrogen gas. While the use of sodium azide and certain other azide-based gas generant materials was in accordance with industry specifications, guidelines and standards, such use could potentially involve or raise potential concerns such as involving the safe and effective handling, supply and disposal of such gas generant materials. Thus, there has been an ongoing need for further improved, safe and effective alternative gas generants, such as composed of an azide-free fuel material and an oxidizer therefor, such as upon actuation react to form or produce an inflation gas for inflating vehicular safety restraint devices.
In view of this need, significant efforts have been directed to minimizing or avoiding the use of sodium azide in automotive airbag inflators. Through such efforts, various combinations of non-azide fuels and oxidizers have been proposed for use in gas generant compositions. These non-azide fuels are  generally desirably less toxic to make and use, as compared to sodium azide, and may therefore be easier to dispose of and thus, at least in part, found more acceptable by the general public. Further, non-azide fuels composed of carbon, hydrogen, nitrogen and oxygen atoms typically yield all gaseous products upon combustion. As will be appreciated by those skilled in the art, fuels with high nitrogen and hydrogen contents and a low carbon content are generally attractive for use in such inflatable restraint applications due to their relatively high gas outputs (such as measured in terms of moles of gas produced per 100 grams of gas generant material).
Lund et al., U.S. Pat. No. 5,197,758, issued 30 Mar. 1993, relates generally to gas generant compositions or propellants which comprise a non-azide fuel which is a transition metal compound of an aminoarazole. As disclosed, preferred transition metal complexes are zinc and copper complexes of 5-aminotetrazole and 3-amino-1,2,4-triazole, with the zinc complexes most preferred. The compositions are also disclosed as including a conventional oxidizer such as potassium nitrate or strontium nitrate.
Most oxidizers known in the art and commonly employed in gas generant compositions are metal salts of oxygen-bearing anions (such as nitrates, chlorates and perchlorates, for example) or metal oxides. Unfortunately, upon combustion, the metallic components of such oxidizers typically end up as a solid and thus reduce the relative gas yield realizable therefrom. Consequently, the  amount of such oxidizers in a particular formulation typically affects the gas output or yield from the formulation. If oxygen is incorporated into the fuel material, however, less of such an oxidizer may be required and the gas output of the formulation can be increased.
In addition to low toxicity and high gas outputs, preferred gas generant materials are desirably relatively inexpensive, thermally stable (i.e., desirably decompose only at temperatures greater than about 160° C.), and have a low affinity for moisture.
Moreover, in addition to the above-identified desirable properties and characteristics, gas generant materials for use in automotive inflatable restraint applications must be sufficiently reactive such that upon the proper initiation of the reaction thereof, the resulting gas producing or generating reaction occurs sufficiently rapidly such that a corresponding inflatable airbag cushion is properly inflated so as to provide desired impact protection to an associated vehicle occupant. In general, the burn rate for a gas generant composition can be represented by the equation (1), below:rb=k(P)n  (1)
where,
rb = burn rate (linear)k = constantP = pressuren = pressure exponent, where the pressure exponent is theslope of a linear regression line drawn through a log-logplot of burn rate versus pressure.
Guanidine nitrate (CH6N4O3) is a non-azide fuel with many of the above-identified desirable fuel properties and which has been widely utilized in the automotive airbag industry. For example, guanidine nitrate is commercially available, relatively low cost, non-toxic, provides excellent gas output due to a high content of nitrogen, hydrogen and oxygen and a low carbon content and has sufficient thermal stability to permit spray dry processing. Unfortunately, guanidine nitrate suffers from a lower than may be desired burn rate. Thus, there remains a need and a demand for an azide-free gas generant material which may more effectively overcome one or more of the problems or shortcomings described above.
Commonly assigned Mendenhall, U.S. Pat. No. 6,550,808, issued 22 Apr. 2003, the disclosure of which is fully incorporated herein by reference, relates generally to gas generant compositions which desirably include or contain guanylurea nitrate (also known as dicyandiamidine and amidinourea). In particular, guanylurea nitrate advantageously has a relatively high theoretical density such as to permit a relatively high loading density for a gas generant material which contains such a fuel component. Further, guanylurea nitrate exhibits excellent thermal stability, as evidenced by guanylurea nitrate having a thermal decomposition temperature of 216° C. In addition, guanylurea nitrate has a large negative heat of formation (i.e., −880 cal/gram) such as results in a cooler burning gas generant composition, as compared to an otherwise similar gas generant containing guanidine nitrate. 
While the inclusion or use of guanylurea nitrate in gas generant materials can serve to minimize or avoid reliance on the inclusion or use of sodium azide or other similar azide materials while providing improved burn rates and overcoming one or more of the problems, shortcomings or limitations such as relating to cost, commercial availability, low toxicity, good thermal stability and low affinity for moisture, even further improvement in the burn rate of gas generant formulations may be desired or required for particular applications.
Basic copper nitrate (Cu(NO3)2.3Cu(OH)2) (sometimes referred to herein by the notation “BCN”) has or exhibits various properties or characteristics including, for example, high gas output, density and thermal stability and relatively low cost such as to render desirable the use or gas generant composition inclusion thereof as an oxidizer. The use of such basic copper nitrate or related materials has been the subject of various patents including Barnes et al, U.S. Pat. No. 5,608,183, issued 4 Mar. 1997; Barnes et al, U.S. Pat. No. 5,635,688, issued 3 Jun. 1997, and Mendenhall et al., U.S. Pat. No. 6,143,102, issued 7 Nov. 2000, the disclosures of which are fully incorporated herein by reference.
In practice, it is generally desired or required that inflators for inflatable restraint systems be able to supply or provide inflation gas in predetermined mass flow rates. The gas mass flow rate resulting upon the combustion of a gas generant composition is typically a function of the surface area of the gas generant undergoing combustion and the burn rate thereof. Unfortunately,  a limitation on the greater or more widespread use of basic copper nitrate in such gas generant compositions is that basic copper nitrate-containing gas generant compositions may exhibit or otherwise have associated therewith undesirably low or slow burn rates. In practice, the normal or typical burn rates associated with such gas generant compositions can act to restrict the use of such gas generant compositions to those applications wherein faster burn rates are neither required nor desired. For example, such low or slow burn rate compositions may be unsuited for various side impact applications where more immediate generation or supply of inflation gas may be required or desired.
For some inflator applications, a low gas generant formulation burn rate can be at least partially compensated for by reducing the size of the shape or form of the gas generant material such as to provide the gas generant material in a shape or form having a relatively larger reactive surface area. In practice, however, there are practical limits to the minimum size of the shape or form, such as a tablet, for example, to which gas generant materials can reproducibly be manufactured and increased burn rates may be needed for particular applications which require a higher inflator performance.
Thus, there is a need and a demand for methods or techniques for increasing the burn rate of a gas generant formulation as well as for non-azide based gas generant formulations having desirably increased or elevated burn rates. 